Topic: replacing stock exhaust cone w/ straight pipe?

[JohnnyTremendous.]

My OEM exhaust cone on my 76 cb750a is rusted out on the bottom and I'm considering removing it from the header and adding a home made, unbaffled straight pipe to replace it.  Will I run into issues with jetting or back pressure, do you figure?

[TwoTired]

Yep

[Sporkfly]

The reason Harleys don't have acceleration and Hondas do has quite a bit to do with the fact that Harleys have straight pipes and therefore no back pressure.

Yeah, you'll get problems running it like that. I'm thinking maybe even some burnt exhaust valves? Somebody else will have to confirm that.

[Pinhead]

The reason Harleys don't have acceleration and Hondas do has quite a bit to do with the fact that Harleys have straight pipes and therefore no back pressure.

Why do drag racers run straight headers without mufflers?

The pipes completely rotted off on my '79 650 (and were removed) and there seems to be an appreciable improvement in low-end torque, but above 4000 power has taken a hit. Strangely enough the engine will now idle smoothly at 900 rpm whereas before I had to have it set to 1300. Also, I can pull away in 1st at idle, whereas before there was absolutely no possible way to take off below 1500.

I'm gong to try to input my engine specs into Pipemax to see how well the stock header/collector system is tuned for the operating band of the engine. Mine being a 650, however, will make it quite a bit different than your 750.

The way the mufflers are designed on my 650, it seems that the "collector" extends into the muffler about 6 inches, and then is dumped into a "termination box" (a large change in CSA and volume that acts as "open air").

[Pinhead]

BTT. I put a 2" slip-on straight pipe on the end of my cut-off collectors. Wow, what a difference.  This 650 will really get up and move!  I've got all of the advantages posted above with regards to low-end torque, but gained even more on top. However I don't know why, with all of the mods that I've done thus far, I haven't had to touch the carbs...

And the sound is just  .

[billboy2000]

BTT. I put a 2" slip-on straight pipe on the end of my cut-off collectors. Wow, what a difference.  This 650 will really get up and move!  I've got all of the advantages posted above with regards to low-end torque, but gained even more on top. However I don't know why, with all of the mods that I've done thus far, I haven't had to touch the carbs...

And the sound is just  .

Is this still working out for you.  I have the same setup, and it seems pretty smooth across the whole range.

[KingCustomCycles.com]

Jetting is probable, noise is a certainty, horsepower is subjective without a dynamometer.  I run loud and proud with wide open pipes all the time, don't get me wrong. If it isen't setting off car alarms when you idle by it, it should. 

[NickC]

The reason Harleys don't have acceleration and Hondas do has quite a bit to do with the fact that Harleys have straight pipes and therefore no back pressure.

Yeah, you'll get problems running it like that. I'm thinking maybe even some burnt exhaust valves? Somebody else will have to confirm that.

They also are completely different engines. Comparing apples and oranges when it comes to exhaust and backpressure.
The lower the backpressure the better. Yes, there are usually other mods that need to happen with a strait exhaust in order to perform right.
Running lean burns exhaust valves, not lack of backpressure. 

[speedy gonzalais]

ive been thinkin strait thru for a bit now then reading this it hit me, All bikes imported to the uk have to be restricted they do this by putting rings in the pipes limiting the amount of air that can escape thru the pipes, this limits the hp and rpm so i guess romoving the back pressure can improve on performance.

[speedy gonzalais]

ive been thinkin strait thru for a bit now then reading this it hit me, All bikes imported to the uk have to be restricted they do this by putting rings in the pipes limiting the amount of air that can escape thru the pipes, this limits the hp and rpm so i guess romoving the back pressure can improve on performance.

But i think to make any big difference you would prob need to modify the air intake to get more in then add some more fuel to the mix but the engine wont last so long not a standard one any way

[Pinhead]

BTT. I put a 2" slip-on straight pipe on the end of my cut-off collectors. Wow, what a difference.  This 650 will really get up and move!  I've got all of the advantages posted above with regards to low-end torque, but gained even more on top. However I don't know why, with all of the mods that I've done thus far, I haven't had to touch the carbs...

And the sound is just  .

Is this still working out for you.  I have the same setup, and it seems pretty smooth across the whole range.

Yep, working beautifully. Mileage is 55mpg at 70mph, power is up all around, idle is smooth at 900 rpm and I can shift out at 3k. I'm taking advantage of the added torque with a 17T/37T sprocket combination.

[Nathaniel]

This is loco leons on www.dotheton.com You should look for him on there and ask him how he got it all set up.